This invention relates to dielectric resonant oscillators, that is, to oscillators of the kind comprising a signal path coupled electromagnetically to an adjacent resonant cavity constituted by a body of dielectric material. The dielectric body, herein referred to as a "puck" has dimensions and a dielectric constant which together determine the resonant frequency (or frequencies) of the `cavity` and thus the oscillation frequency. The dielectric puck may be mounted on a printed circuit board by adhesion to the substrate, closely adjacent to a stripline conductor constituting the above signal path.
A typical operating frequency for such an oscillator would be 10 GHz and a typical application would be as a local oscillator in a receiver of a satellite transmission. The X-band transmission is thus converted to a relatively low intermediate frequency of, say, 1-2 GHz. The converter circuitry is commonly formed on printed circuit board employing stripline conductors and both `printed` and discrete components. While the term "printed circuit" is used in this specification for convenience, it will be appreciated that the actual method of forming stripline conductors and like circuitry is not directly relevant to the invention and the term is thus to be interpreted broadly.
There are several problems associated with temperature effects on the circuit components. One problem concerns the temperature sensitivity of the physical features of the circuitry e.g. screw expansion (where a tuning screw is used), board expansion, board dielectric change, and the consequent change of operating frequency with temperature. This can be largely compensated by a suitable permittivity temperature characteristic of the puck material, of which a range is available. Thus a puck material is available having a `frequency compensation` characteristic of 9 parts per million/.degree. C., the dielectric constant of the puck material changing with temperature in a direction such as to oppose the effect of circuit temperature on frequency. At a frequency of 10 GHz this would provide compensation of about 7 MHz over a temperature range of -20.degree. C. to +60.degree. C.
However, this compensation facility is modified by the presence of the substrate and its temperature/dielectric constant characteristic. The variation of board dielectric constant E.sub.r with temperature is illustrated, for a PTFE material, in FIG. 1 of the accompanying drawings.
The substrate temperature sensitivity makes its presence felt because the electric field in the puck couples with the substrate so that the puck and substrate tend to form a single resonant entity. The frequency drift is therefore determined partly by the substrate characteristic.
The dielectric constant of the puck is high, e.g. 35, whereas that of the substrate is perhaps 2, for PTFE, up to 10 for alumina. Care must be taken in the choice of substrate material so as not to degrade the circuit Q, high values of which are obtained with ceramic (e.g. alumina) or PTFE based low loss materials.
It has been proposed to mount the puck off the substrate on a ceramic pedestal but this involves complex assembly procedures.